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n6             PRACTICAL ORGANIC CHEMISTRY

hour or two (or preferably overnight), the hydrochloric
excess of alcohol and water are expelled by evacuating-
flask and distilling in vacito on the water-bath. The
maining half of the alcohol is added to the residue, and
mixture again saturated in the cold with hydrochloric acid &'**s-
After standing, the acid, alcohol and water are removed <*s
before, and the residue fractionated from an oil or metal batli in
vacua. The ethyl tartrate distils as a clear viscid liquid. After
a second distillation in vacua the substance is pure.
At ii mm. it boils at 155°.
„    20     „       „       „        164°.
The yield is 80 per cent of the theory.    See Appendix, p. ^6 2.
Determination of Rotatory Power.—The rotatory power
of ethyl tartrate, which is an optically active substance, is
determined by means of a polarimeter. One of these instru-
ments known-as Laurent's polarimeter is shown in Ifi&s.
71 and 72.
The monochromatic light of a sodium flame is used in these
determinations and is obtained by  suspending  in a  Bun sen
flame a platinum wire basket containing fused sodium chloride
or the more volatile bromide.    The latter gives.a brighter flmnc,
but the basket requires replenishing more  frequently.        The
light from the flame passes  through  a  cell  B,  containing    a
solution of potassium bichromate (or a crystal of this substixri <'*<•')i
which deprives it of blue or violet rays.    It then passes thi-ough
the polarising nicol prism P.   A plate of quartz cut parallel    to
the optic axis covers half the opening D, and is of such a   t "hick-
ness that it produces a difference of a half-wave length   (or   an ,
exact odd multiple of a half-wave length) between the two    r*iys, ;
which it gives by double refraction.    The light then    passes .<
through the substance placed in the tube T and entering-   a.t  K f
strikes the analysing nicol N.    The telescope OH  is focus set I cm
the edge of the quartz plate at D.    When N is turned, a pointer
moves over the graduated circle C and its position can be    read |
by means of the lens L.                                                                  )
The Theory of the Instrument may be explained   as <
follows :—If, after passing through the nicol P, the plant*   of
vibration is in the direction OB, Fig. 73 a, then in the half of   the \
field to the right, uncovered by the quartz plate, it passes on  un- f